There is known a device for preparing a gradient solution for a high-pressure liquid chromatograph comprising vessels containing eluents of various compositions communicating by way of pipes through a switch-over unit with displacement chambers of ram-type pumps and through a mixer with the inlet of the chromatographic column (U.S. Pat. No. 3,712,513).
In this known device the number of pumps is dictated by the number of components in the gradient eluting solution; each such pump delivering under pressure the respective component at a rate determined by a control computer. The computer also issues signals to solenoid-operated cut-off values in the switch-over unit.
However, this prior art device fails to sufficiently accurately reproduce the shape of the gradient curve due to the low frequency of actuation of the solenoid-operated valves sealed to withstand the high working pressure of the chromatograph. Another disadvantage is that such a thorough sealing affects the reliability and shortens the service life of the valves.
Also, the aforedescribed device is not applicable for preparing a gradient solution for microcolumn chromatographs. The entire volume of the gradient solution required for carrying out complete analysis in the microcolumn chromatograph amounts to approximately 20-60 mkl, whereas the volume of the displacement chamber of the ram-type pump is never less than 30-80 mkl. At the same time, it is impossible to prepare a gradient solution characterized by a curve of intricate shape by one or two strokes of the pump.
There is also known a device for preparing a gradient solution for a high-pressure liquid chromatograph comprising vessels containing eluents communicating by pipes through a switch-over unit with the displacement chamber of a piston pump and through a mixer with the inlet of the chromatographic column (c.f., U.S. Pat. No. 4,437,812).
This device includes only one pump the displacement chamber of which also functions as a mixer. The switch-over unit comprises distribution valves to proportion the components of the gradient solution, and two cut-off valves acting to either separate the chromatographic column from the pump at the stage of preparation of the gradient solution, or to separate the pump from the proportioning valves at the point of forcing the mixture to the chromatographic column. The proportioning valves are sealed to withstand low pressure, since they are actuated only during the suction cycle of the pump, the cut-off valves being capable of withstanding high pressure.
The proportioning valves in this prior art device are sufficiently fast-acting; however, the frequency of reciprocations of the ram-type pump is rather limited, and fails to ensure a sufficiently accurate reproduction of a set shape of the gradient curve.
As the previously described device, this apparatus is not applicable for use in microcolumn chromatographs due to the substantial volume of the displacement chamber of the piston pump.